Media captionJohn Longworth from the British Chambers of Commerce backs HS2, but Mark Littlewood from the Institute of Economic Affairs says it is a waste of money

"A £15bn annual boost to the economy. With the North and Midlands gaining at least double the benefit of the south."

Dismissing "scare stories" over the budget, he maintained that it would remain £42.6bn, with a contingency fund of £14.4bn.

But earlier this week, the Public Accounts Committee (PAC) criticised the scheme, saying: "So far, the Department [of Transport] has made decisions based on fragile numbers, out-of-date data and assumptions which do not reflect real life."

The committee also said there was no evidence the line would help the growth of regional cities and would instead draw even more business to London.

KPMG's report was commissioned by HS2 Ltd, which is a non-departmental public body wholly owned by the Department for Transport.

Report author Richard Threlfall, KPMG's head of infrastructure, said: "What I hope this work will do is put some new evidence into the debate, because what we've seen over the last few months is an awful lot of opinions and to be honest not a lot of evidence on the ground as to what effect this will have.

"And what our report shows, beyond any reasonable doubt, is that HS2 will deliver massive benefits to the UK economy."

'Creaking'

John Longworth, director general of the British Chambers of Commerce, told the BBC: "Businesses up and down the country are absolutely convinced that this is a really important infrastructure project. It is one of the key infrastructure projects for the UK."

Analysis

By Richard WestcottBBC transport correspondent

This report tots up the benefits in a new way.

It analyses how better transport links have apparently fuelled business and productivity across various different cities, then applies that to HS2.

Saving time still plays a part, but it's a smaller part.

Instead, there's more importance placed on all those extra seats this scheme provides, not just on the new high speed trains, but on the rest of the rail network too (all those extra commuter services we're being promised).

Critics say the government is just moving the goalposts in a desperate bid to make its project look better.

He argued that simply renovating existing rail lines that were "creaking at the seams" would cause "chaos".

The latest study was commissioned by HS2 Ltd, the company responsible for developing and promoting the project.

It says Birmingham's economy could be boosted between 2.1% and 4.2% a year, while Manchester would benefit between 0.8% and 1.7%.